1. Field of the Invention
This invention relates generally to data communications. More particularly, the invention relates to high-speed modems designed to provide transmission speeds of 56 kbps in both downlink and uplink directions.
2. Description of the Related Art
In the field of wireline telephone modems, one type of modem is known as a “V.90 modem.” “V.90” references an international standard (recommendation V.90) agreed upon by the International Telecommunications Union (ITU-T). A V.90 modem is a specific member of a class of modems known as “PCM modems.” In fact, before the V.90 standard was agreed upon, V.90 was known colloquially as “V.PCM.” “PCM” is an acronym which stands for “pulse code modulation.” PCM was originally developed to transmit digitized speech signals through a telephone network such as the public switched telephone network (PSTN). PCM as used in the PSTN is defined in the ITU-T G.711 standard. In the United States, PCM involves sampling a speech signal, encoding each sample to roughly 13 bits of linear resolution, and then logarithmically companding and re-encoding the sample to an 8-bit mu-law code. PCM coding involves a well-known and accepted form of nonlinear logarithmic compression whereby smaller signal amplitudes are represented at greater resolution than larger amplitudes. European countries use a different logarithmic compression law known as A-law. PCM encoding techniques provide a way to efficiently encode analog speech waveforms. This is because speech waveforms have a wide dynamic range and the human hearing system itself performs logarithmic signal processing.
The ITU-T's V.90 standard represents an accepted technology used to transfer information between an analog telephone subscriber and a network server such as an Internet service provider (ISP). V.90 compliant modems are asymmetric in that different signal protocols are used in the uplink and downlink communication directions. In connection with V.90 modems, the term “uplink” corresponds to the communication channel from the analog telephone subscriber to a digital network server. The “downlink” corresponds to the communication channel from the digital network server to the analog telephone subscriber. The V.90 recommendation makes the assumption that communication must pass through a standard network interface comprising a PCM codec. A PCM “codec” is a converter which converts an analog signal to a PCM signal (COder) and also converts a PCM signal to an analog signal (DECocer). A coder and a decoder are thus collectively called a “codec.” The codec may be viewed as a communication impairment, because at this analog interface, noise and distortion are added to an otherwise pure digital communication path capable of carrying 64 kbps in both the uplink and downlink directions. Certain types of digital impairments such as robbed-bit signaling may also reduce sustainable data rates in the digital network to 56 kbps or lower.
FIG. 1 illustrates a network configuration as is present in a V.90 oriented communication system. A digital network server 102 typically represents an ISP or a database server which is connected to a digital network 110. The digital network 110 typically involves a network such as the PSTN and the digital network server 102 typically connects to the digital network 110 via a digital connection such as an ISDN line or a T1 line. The digital network server 102 thereby couples to the digital network 110 via a digital modem 105. No codec is needed to coupling the digital modem 105 to the digital network 110. The digital modem 105 transmits and receives digital information across the digital network 110. At the edge of the digital network 110 is a network interface 115 typically implemented using a line card. The network interface 115 includes a codec 117 and a line interface circuit 130. The codec 117 accepts a digital stream of information from the digital network 110 into a digital-to-analog converter (DAC) 120. The DAC 120 produces an analog output voltage based on the decompressed value of a mu-law code applied to its input. This data stream passing through the DAC 120 is said to travel in the “downlink” direction. The output of the DAC 120 feeds to a low pass filter (LPF) 125 which is also part of the codec 117. The LPF 125 attenuates high frequency components in the downlink signal to perform reconstruction and line filtering as is known in the art. The LPF 125 may also attenuate a small portion of the low frequency band, for example from 0 Hz to 250 Hz. The output of the LPF 125 feeds a line interface circuit 130 which in this example is illustrated as a hybrid circuit. The hybrid circuit 130 provides a four-wire to two-wire conversion and is typically implemented using transformers and electronic amplifiers. The hybrid circuit 130 interfaces to an analog subscriber line 137. The analog subscriber line 137 involves a telephone line and the electrical signaling applied thereto. The downlink signal as converted by the DAC 120 and filtered by the LPF 125 is coupled by the hybrid circuit 135 onto the subscriber line 137. An analog “uplink signal” is also coupled via the hybrid circuit 135 from the subscriber line 137. The analog uplink signal is passed via the hybrid circuit 130 to an LPF 135 which performs antialiasing. In addition to attenuating high frequency components, the LPF 135 may also attenuate a small portion of the low frequency band, for example from 0 Hz to 250 Hz. The output from the LPF 135 is passed to an analog-to-digital converter (ADC) 140. Besides converting the filtered analog uplink signal to digital, the ADC typically logarithmically compresses the data according a compression curve such as the mu-law curve. Both the LPF 135 and the ADC 140 are also typically implemented as a part of the codec 117 which is itself implemented on a single integrated circuit die. Because the ADC 140 is found within the network interface 115, the ADC 140 is also referred to herein as the “network-interface ADC.” Similarly, the DAC 120 is also referred to herein as the “network-interface DAC.”
Signals are coupled from the subscriber line 137 to an analog modem 145 via a line interface circuit 150. The subscriber line 137 is therefore coupled via a network-side coupling to the network interface 115 and via a subscriber-side coupling to the subscriber modem 145. A transmitter 160 generates a digital representation of an analog uplink to be sent over the subscriber line 137 back to the network interface 115. In present day V.90 systems, the digital representation of the analog uplink signal is defined by the physical layer signal specification of the ITU-T recommendation V.34. The digital signal output from the transmitter 160 is coupled to a DAC 162. Typically the DAC 162 is a high precision linear converter and is followed by a bandpass reconstruction filter with a passband in the range from approximately 250 Hz to 3500 Hz (not shown). This filtered analog uplink signal is coupled onto the subscriber line 137 via the line interface circuit 150. The analog downlink signal sent by the network interface 115 is coupled from the subscriber line 137 via the line interface circuit 150. Although not shown, the line interface circuit 150 also typically includes a receive-antialiasing filter. The received analog downlink signal is then digitized by an ADC 152 which typically samples its input at 16000 Hz and quantizes each sample to 16 bits of linear resolution. This received and digitized downlink signal is then combined with the digital representation of the analog uplink signal using an echo canceller 170. The echo canceller 170 cancels an echo component which leaks from the transmit circuit 160's output into a receiver circuit 155's input via the line interface circuit 150. The receiver circuit 155 performs equalization and other processing to condition the received downlink signal prior to making decisions to regenerate the transmitted downlink bit stream.
In V.90 compliant systems, the downlink signal is derived from a digital data stream in the digital modem 105 and is transmitted digitally all the way through the digital network 110 until it reaches the codec 117 in the line card 115. The DAC 120 then directly converts the downlink signal into a pulse train. The pulse train generated at the output of the DAC 120 is shaped by the logarithmic decompression law (e.g., mu-law or A-law) employed in the codec 117. The pulse train is then filtered by the LPF 125 and is coupled via the hybrid circuit 130 onto the subscriber line 137 for receipt by the analog subscriber modem 145. This type of pulse transmission system is defined in the ITU-T V.90 recommendation which is incorporated herein by reference. A similar pulse oriented transmission system (referred to as the “Townshend system” hereinafter) is described in U.S. Pat. No. 5,801,695 issued to Brent Townshend. U.S. Pat. No. 5,801,695 is hereby incorporated herein by reference. In such systems, a high-speed pulse oriented modulation protocol is used in the downlink direction to achieve data rates as high as 56 kbps. Theoretically data may be transmitted from the digital server 102 to the analog subscriber modem 145 at as much as 64 kbps, but due to digital impairments and especially FCC power restrictions, the highest rate practically achievable is 53 kbps. If line conditions are poor, still lower rates may result. In the present application, the term “high speed” thus refers to a connection speed higher than 33.6 kbps and less than or equal to 64 kbps. For example, a 53 kbps connection is a “high-speed” connection as defined herein.
The Townshend system and the V.90 systems are both asymmetric. That is, different physical layer communication protocols are defined in the downlink and the uplink directions. While the downlink uses a fast 56 kbps pulse modulation scheme, the data rate on the uplink uses a trellis coded modulation oriented physical layer signaling protocol as defined by the ITU-T V.34 recommendation. The V.34 physical layer protocol accommodates data rates no higher than 33.6 kbps. Hence in present day asymmetric 56 kbps modem architectures, the uplink data rate is on the order of 60% as fast as the downlink speed. In client-server applications such as those involving an Internet browser coupled to an Internet server, most data traffic travels in the downlink direction, so the slower uplink connection is deemed to be acceptable. Still, the acceptability of this solution is based on the assumption that the analog subscriber modem 145 is communicating with the digital network server 102, e.g., an ISP. It would be desirable to have a modem which could transmit data symmetrically or at least nearly symmetrically at 56 kbps in both the uplink and downlink directions. In addition to faster upload speeds, this would enable 56 kbps data rates to be achieved in subscriber-to-subscriber applications such as fax machines and videophones.
More generally, the “uplink direction” may be defined as the signal path direction from the analog subscriber modem 145 to the network interface 115. The “downlink direction” may be defined as the signal direction from the network interface 115 to the subscriber modem 145. This more general definition is useful in the context of the present invention whereby symmetric communication channels between subscriber modems may be established. With this definition, each subscriber modem transmits in the uplink direction to the network interface 115 via the subscriber line 137. Also, each subscriber modem receives from the downlink direction from the network interface 115 via the subscriber line 137. In a peer-to-peer connection a first subscriber modem transmits an uplink signal which is received by a second subscriber modem as a downlink signal. When the subscriber modem 145 is connected to the digital server 102, the subscriber modem still transmits in the uplink direction and receives from the downlink direction. The digital modem receives from the uplink direction and transmits in the downlink direction.
Attempts have been made to provide high-speed uplink connections. U.S. Pat. No. 5,394,437 to Ayanoglu et. al. (the “Ayanoglu system” hereinafter) defines an analytical solution which allows data rates as high as 56 kbps in the uplink direction. The main idea is to transmit a carefully pre-equalized pulse train from the analog subscriber modem in the uplink direction. The pre-equalization is designed to ensure the input to the network-interface ADC 140 samples data values which are substantially equal to the ADC 140's quantization points. An improvement to this system is provided in U.S. Pat. No. 5,406,583 to Dagdeviren. The “Dagdeviren system” as defined therein provides a means to condition a 56 kbps-uplink communication path from within the digital communication network. The main idea is to apply a code conversion within the digital network 110 to control voltage levels so as to compensate for echo effects which limit the practical application of the Ayanoglu system. No echo model is discussed in the Ayanoglu patent. As pointed out in the Dagdeviren patent, echo problems seriously limit the performance and practical usability of the Ayanoglu system. These echo problems prohibit current PCM modem technology such as defined by the V.90 standard, the Townshend system, and Ayanoglu system from being able to provide a high-speed pulse communication path in the uplink direction. Unfortunately, to correct the Ayanoglu system using the Dagdeviren system, costly call processing services involving real-time code conversions need to be integrated into the existing PSTN infrastructure. Hence deployment of the Dagdeviren system has been limited to date.
Echo cancellers are well known in the art and have been available for several decades. However, echo cancellers rely on knowledge of the transmitted signal. The echo limiting the uplink data rate is illustrated as echo path 127 in FIG. 1. The echo path 127 couples energy from the high-speed downlink data path back into the uplink data path via the line interface 130. Due to the nonlinear compression laws used in the network codec 117, a large signal pulse value transmitted in the downlink direction may give rise to an echo which causes a small uplink value to be lost. This is because the small uplink value will need to be quantized according to a closely spaced set of quantization points. When a large echo component is also present, the echo-plus-uplink signal sample will be quantized using a portion of the quantization grid where quantization points are more distantly spaced. This results in a loss of precision because the echo-plus-uplink signal will be quantized to the value of the larger echo component. When the echo is eventually cancelled in the digital modem 105, no bits of precision will remain from which to extract the uplink signal value. This effect limits the achievable data rate in the uplink direction.
A technical difficulty which has limited uplink data rates is the inability of prior art subscriber modems and PCM codecs to cancel an echo at the input to the network-interface ADC 140. Echo cancellation traditionally occurs within the digital modem 105 and the analog subscriber modem 145. These modems are located at the telephone connection's endpoints. Prior art adaptive echo cancellers can cancel an echo as seen from the modem endpoints, but not at the inaccessible point in the network codec 117 prior to the codec's ADC 140. Also, intersymbol interference needs to be minimized so that it does not impair the ability of the network-side codec to sample a pulse signal at its proper values. Moreover, symbol timing recovery at the network codec may drift from the sample clock used within the analog subscriber modem 145. Hence it is difficult at best to generate signals which will be locked to the sample clock and quantization levels of the network-interface ADC 140.
It would be generally desirable to transmit data at a higher speed in an uplink direction across a telephone line terminated at the network side by a PCM codec. For example, this would provide improved upload speeds and enable high-speed symmetric communication channels for use in applications such as facsimile and video telephony. It would be desirable to have a low cost enhanced network codec which enables highly reliable symmetric 56 kbps modem connections. It would also be desirable to have an echo canceller which could be built into one or more modem endpoints to cancel an echo at the network codec 117, thus allowing data to be transmitted at 56 kbps in both the uplink and downlink directions without the need to modify the network codecs located within the telephone system. Moreover, it would be desirable to have systems and training procedures to allow digital and/or subscriber modems to cooperatively train by sending probing signals and adjusting their parameters so as to provide symmetric 56 kbps connections. New forms of adaptive signal processing structures are needed to configure and operate a remote-echo canceller to cancel an echo as measured at the input to a network codec's ADC using signal processing only within the modem endpoints.